Basically, in optical communication networks information is transmitted by use of a light source (transmitter), an optical fiber and an optical receiver. Typical wavelengths used for optical communication are wavelengths in the range of 850 to 1650 nm, and particularly laser diodes with a wavelength in the range of 850 nm, 1300 nm and 1550 nm are used as light sources.
In wavelength-division-multiplexing (WDM) optical communication systems, information is transmitted simultaneously by a set of laser sources, each generating coherent light with a different wavelength (optical communication channels). Since the bandwidth of optoelectronic transmitters and receivers is limited, narrow channel spacing (typically 1.6 nm) is needed to increase the transmission capacity by using a multiplicity of communication channels. Particularly, in WDM systems there is a need to adjust the wavelength of each laser source very precisely to avoid channel interferences at narrow channel spacing etc.
To adjust the wavelength of the signals of a laser source, it is known to use an expensive and very precisely measuring wavemeter comprising a well-adjusted and complex mechanical arrangement. The wavelength of the signals of the laser source is measured, compared with a desired value by a controller, such as a PC, and the wavelength of the signals of the laser source is automatically adjusted to the desired wavelength.
WO 95/02171 discloses a Fourier-transform spectrometer which contains a birefringent optical component, removing the need for a Michelson interferometer used in conventional instruments. A suitable birefringent element, such as a Wollaston prism, is used to introduce a path difference between two light polarisations. Use of an extended light source so that all areas of the birefringent component are illuminated simultaneously ensures that different positions on the birefringent component correspond to different path differences between the two polarisations. A Fourier-transform of the resulting interferogram at the detector results in the spectral distribution of the input light being obtained. The use of an extended light source permits a Fourier-transform spectrometer with no moving parts to be achieved.
P. Juncar et al: "A new method for frequency calibration and control of a laser", OPTICS COMMUNICATIONS, Vol. 14, No. 4, August 1975, Amsterdam NL, pages 438-441, XP002041763 discloses a method for high-precision measurement of the wavenumber of monochromatic radiation emitted by a single-mode tunable laser. The described apparatus allows a direct measurement of the wave number, and serves as a reference for the stabilization and piloting of the laser frequency.
WO 95/20144 discloses an optical wavelength sensor which consists of a wedge shaped Fabry Perot etalon which exhibits resonance for different optical wavelengths across its width, and an array of detectors that detects the spatial disposition of resonant peaks which occur in the etalon, for comparison with stored peak patterns in a processor, so as to determine the spectral content of the incident light from an optical fiber.
WO 95/10759 discloses a spectal wavelength discrimination system and method which allow the wavelength of a beam of radiation to be accurately determined. The system comprises an optical system for gathering and directing received radiation; a wavelength selective beamsplitter, termed a Linear Wavelength Filter, for directing predetermined fractions of the beam at each wavelength into each of two output beams; a detector for receiving each output beam to sense the intensity of each output beam; and a computer for determining the wavelength of the received radiation. Intensity measurement of the output beams and selected system parameters, including the beamsplitter spectral characteristics and detector sensitivity characteristics are unsed in a special algortihm for performing Fourier based wavelength-dispersive analysis. The unique solution of the Fourier based analysis is the wavelength of the beam of radiation.
It is an object of the present invention to provide an inexpensive wavemeter which is insensitive to a large extent against adverse influences under conditions to be met in practice.
Another object of the invention is to provide an inexpensive arrangement for the automatic adjustment of the wavelength of the optical signals of a laser source.